To follow up on our whale adaptations series this blog post will focus on how whales, as warm-blooded mammals, can stay warm while living in water—especially cold-water environments. Just like us, whales are mammals and maintain a steady internal body temperature regardless of their environment. In fact, their body temperature is close to our own—varying from about 97 to 100 degrees. However, water conducts heat away from the body 24.5 times faster than air, making heat loss a big issue for any mammal spending time in the water. This is the reason we feel colder in water, and why we can tolerate colder air temperatures than we can water temperatures.
In order to reduce heat loss, whales have three main adaptations: reducing the body’s surface area to volume ratio, using their thick blubber layer as an insulator, and retaining heat through counter-current heat exchange.
It is important for whales to have a low surface area relative to their total body volume. I think pictures might help explaining this better:
Clearly, mice and elephants have very different body sizes and much different body volumes—the amount of space occupied by their body matter. But what I’d like to illustrate is that relative to their size, the mouse has a much higher surface area where heat could be lost to the external environment. If both of these animals were exposed to cold weather, both would eventually lose heat to the environment. The mouse, however, would lose heat much faster since a greater percentage of its total body volume is being exposed to the surface. In order to preserve heat, it is more beneficial to have a larger volume compared to your body surface, so there are fewer opportunities for heat loss. This helps to explain why marine mammals tend to be so large, as it is beneficial for them to have the smallest relative surface area in contact with the water. And it’s easier for whales to attain large sizes because they don’t have to deal with the full effects of gravity like land mammals.
The second main way whales stay warm is blubber. Although fur is a good insulator for terrestrial mammals, it would not be as successful for whales for a couple of reasons. Fur works as an insulator because it traps an insulating layer of air: however, the atmospheric pressure beneath the surface waters causes the air to compress and lose its insulating power. Therefore, whales do not have a protective fur coat like many land mammals or seals and polar bears, and rely instead on their thick blubber to insulate their bodies in cold water. This blubber allows for a very smooth external surface, also reducing hydrodynamic drag. Another way whales reduce both heat loss and drag is to internalize their genitalia, instead of it being external like most terrestrial mammals.
The thick blubber layer not only keeps heat on the inside of the body, but the outermost skin layer is cooled to the same temperature of the surrounding water to further reduce heat loss via conduction. The thickness of the blubber coat varies among species and time of year: for example, humpback whales generally have blubber layers around 6 inches thick, while after their feeding season right whales can have a blubber layer up to 50cm thick! Thicker blubber layers also makes certain species more buoyant. In fact, that’s where the name “right whale” came from; they are so buoyant that they even float when dead and during the whaling days were considered the “right” whale to kill. One of the side effects of being buoyant is that these whales will typically raise their flukes out of the water more often when diving because they need the extra help to propel themselves down into the water column. More streamlined whales, like finbacks or minkes, have blubber layers only several inches thick and rarely need to fluke up when diving.
But this is just one side of the story, what if whales overheat because they are swimming fast, are surface active, are pregnant, or are in warmer water. Whales do not have sebaceous glands and cannot sweat like we can to cool off, so they need a different strategy to be able to dump excess heat…and in order to shed heat, there must be a way to bypass the blubber layer. So while the blubber coat provides great insulation for most of the whales’ body, there are certain areas called thermal windows that lack blubber and are not well insulated. These areas include flippers, dorsal fins, and their flukes, each relatively thin and highly vascularized. We are still not entirely sure what all of the purposes of the dorsal fin are because some whales lack it entirely—but we believe that these thermal windows at times function as a way for whales to shed excess heat.
While the thermal windows are great opportunities for the whales to shed heat if overheating, they do not always want to be losing heat to the environment. One of the issues with these areas is that the blood returning from them is cold and could potentially cold shock the heart. This is prevented by another great adaptation—a system called counter-current heat exchange. The arteries and veins in these tissues are very close together but the blood flows in different directions allowing heat to transfer across membranes. So the heat in warm blood that is leaving the heart will heat up the cold blood that is headed back to the heart from the extremities. This means that the heart is always being pumped with warm blood and it decreases the heat lost to the water in those thermal windows.
Whales also have this counter-current heat exchange in the soft palette of their mouths (that’s the pink in the roof of the upper jaw). When baleen whales are in their cold water feeding grounds, they spend at least half the time with their mouths open, a potential large heat loss. This system helps ensure they are not losing too much heat to their environment and maintains warm blood returning to the brain and heart. Clearly a lot of regulation is required to maintain this complicated process, which makes whales extremely unique at regulating their body temperature.